The present invention relates to shot peening and, more specifically, shot peening wherein the shot mass flow rate and the average velocity of the shot are calculated.
The use of shot peening is relatively well known. In particular, a stream of shot (i.e., particles) is directed at a surface at high velocity. The shot is directed at the surface on a workpiece so as to cause plastic deformation of the surface of the workpiece, often a metal surface. The shot peening is often used to increase fatigue strength, although the process may be applied for other purposes.
Various shot peening devices and techniques have been developed over the years.
Shot peening systems generally have (or can be readily equipped with) mass flow controllers. Such controllers are used to control the flow of shot to the shot peening gun. One common type of mass flow controller for use with shot made from magnetic material has an electromagnet which is pulsed in order to allow passage of a metered amount of shot into a shot peening gun. This common type of mass flow controller uses internal feedback to stabilize the mass flow rate (i.e., the amount of shot metered in a given time). A control may be used to set the mass flow rate to a desired value. A display is often used to indicate the flow rate.
As part of a mass flow controller, or as a separate component, prior shot peening systems have included various shot flow meters which provide an indication of the flow rate of the shot. The shot flow meter might be a magnetic densitometer, an example of which is the Model 260 Shot Flow Meter manufactured by Electronics Incorporated of Mishawaki, Ind.
The sensor of the magnetic densitometer, as typified by the Model 260, is a wire coil wound around a tube through which the shot travels vertically. Basically, the device measures the amount of shot under the coil at a given time by sensing the inductance of the coil. In the length of time it takes a particle of shot to traverse the length of the coil, the shot in the coil is fully replaced by new shot.
Therefore, if
L=coil length (inches) PA1 T=time for shot to pass through coil (sec.) PA1 v=shot velocity (in./sec.) PA1 m=amount of shot inside the coil (lbs.) and PA1 R=shot mass flow rate (lbs./sec.), the mass flow rate of shot through the coil is: EQU R=m/T (lbs./sec.) (1)
and EQU v=L/T (in./sec.) (2)
such that EQU R=mv/L (lbs./sec.) (3)
In order to solve for the mass flow rate R, the coil of the magnetic densitometer of Model 260 is installed in the shot feed line vertically beneath the shot flow control valve. From ballistics, the average velocity v of the freely falling shot in the coil is a known constant.
Since the densitometer measures m and the values v and L are known constants in this configuration, the signal processing section of the flow meter performs equation 3 and develops a signal representative of the mass flow rate R.
The most important process parameters in a shot peening operation are the velocity of individual shot particles and the shot mass flow rate. The flow rate determines how quickly the entire surface will be impacted. If the flow rate is too small for a given exposure time, some areas of the surface will remain untreated after the exposure is over. On the other hand, if the mass flow rate is too large, excessive surface cold work may result in surface damage and increased susceptibility to fatigue. Shot velocity establishes the amount of energy or cold work delivered with each impact which in turn controls the surface profile and depth of the compressive layer. The shot particle energy is one-half of the particle mass times the particle velocity squared. The dependence of this kinetic energy upon the particle velocity makes it clear that the shot particle velocity is an important factor in determining the quality of the shot peening.
Although some measurement techniques have been used in conjunction with the shot peening process, most such prior techniques have been inadequate to conveniently and inexpensively provide an indication of the quality of the shot peening technique. The general absence of simple and inexpensive techniques to measure the quality of shot peening inhibits one's confidence that consistent shot peening results can be obtained.
Further, some shot peening systems are unable to detect a malfunction such as a clogged nozzle or an air leak and take corrective action. This inability to detect malfunctions may result in work pieces going through the process without being shot peened.
A further problem is that some prior techniques require measurement of the mass flow rate adjacent to a shot hopper and at a significant distance from the gun. The problem with this approach is that inaccuracies in measurement may occur due to variations in the shot stream characteristics between the sensor and the gun due to flow instability, leakage, kinking, plugging of the shot hose, or other factors. Depending upon the variations in the shot stream characteristics, such measurement errors may be significant.